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Mixing Valve Fail Safe

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desert_sasquatch
desert_sasquatch Member Posts: 118
edited May 2023 in Plumbing
I'm building a home and the current plan is to use solar to--when it's sunny--heat the DHW tank as hot as I can safely do so. (I think because PEX maxes out at 180 F that's the number in my head but maybe folks have other thoughts.)

Anyhow, such a system would obviously require a thermostatic mixing valve. As this is a private residence I'm not looking to do anything too fancy (ie putting thermostatic mixing valves at each fixture, which would be expensive in terms of maintenance). I'm happy with a single point-of-distribution thermostatic mixing valve.

However I do worry about what might happen if the valve were to fail and allow too much hot water through. I don't want to get burned taking a shower.

So that leads me to my two questions:

1) Will thermostatic mixing valves for DHW always fail closed? That is, is there any chance that a failing valve will deliver more hot water than it should?

2) If that's a possibility is there some kind of failsafe I can install directly after the thermostatic mixing valve to block the DHW line if the temperature goes above a set point?

Probably this is a really basic question and the answer is really basic too, but I have read Idronics 22 and also googled around and couldn't find an answer...so I'm hoping folks here will know.

Thanks!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,426
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    So far as I now, no. Mixing valves of the ordinary sort don't have a fail cold mode. Perhaps @hot_rod will know -- he would if anyone did. But my impression is that when they fail, they fail stuck in position.

    For that matter, most thermal safety valves and thermostatic valves are designed to open at a set temperature, rather than close -- and they too have no preferred failure mode.

    It would, of course, be possible to power a zone valve with an aquastat and a relay to do the job -- but that would need a power supply.

    There is a deeper problem here, though: you are planning to use PEX, which does have an upper temperature rating of around 180. How do you propose to limit the temperature in the collectors and associated piping to that value? If you should, for some reason, have a slowing or failure in circulation, you will get well over that figure in a remarkably short time. This could get interesting, if nothing else.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    desert_sasquatchMad Dog_2
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,331
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    Hi, Sort of a different perspective, but you're right that getting burned in the shower is no good. But shower valves these days can be pressure balanced, thermostatically balanced, and have temperature limiting stops. I'd look into that approach, even if only as backup to a mixing valve.

    Yours, Larry
    Mad Dog_2
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,050
    edited May 2023
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    You want to use a valve that has a ASSE 1017 certification. As I remember learning about this particular certification, the Fail Safe is built into the valve in such a way that the valve can not ever allow water leaving the MIX port to exceed 120°F. I'm not sure that my terminology is accurate, the words Fail Safe may not be exactly in the description or specification of ASSE 1017. All I remember is using that specification on my contracts for hot water system installation. When a customer asked about what that meant, I would explain that it is something like Anti Scald but more specific to the hot water system in a residential situation to make sure that the hot water in the shower was not too hot. I would refrain from writing down Anti Scald or Fail Safe in my contracts. I would only wright down the ASSE 1017 specification, and tell the consumer that this is the valve they need for that type protection.

    (That is lawyer talk for CYA)

    When I started in this trade, the oil fired tankless boilers could possibly have a short burst of up to 200°F water from the coil find its way to the shower. The Watts model 70A tempering valve was the only game in town to make sure that 200° water was mixed with cold water before it left the boiler room. That valve was NOT a fail safe valve. Meaning that when the thermostat spring inside the valve failed, the water could leave the boiler room in excess of 140° and end up in the shower.


    So LOOK for the ASSE 1017 Cert and you should be fine.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    mattmia2Mad Dog_2
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    @Larry Weingarten my thought is just that a mixing valve at every faucet is an added expense.

    I was asking in this thread about thermostatic mixing valve servicing. As I said there

    If I have to replace all of them every 5-8 years it sounds like I could easily be paying a few hundred dollars a year for this in my not-huge (10 point of use valve) home.


    Of course maybe with proper servicing it would be a bit less but it still seems like a significant expense...
  • mattmia2
    mattmia2 Member Posts: 9,735
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    The pex rating is complicated because it is a function of pressure and temp and the spec the code uses is at a much higher pressure than what is in a hydronic system. You likely would see boiling before it would exceed the pressure capabilities in a solar system but you won't find a published specification that says that.
    desert_sasquatchMad Dog_2
  • hot_rod
    hot_rod Member Posts: 22,251
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    The ASSE 1017 is a point of distribution valve listing, it is not a fail safe valve by definition
    The ASSE 1070 is a point of use valve and it is required to fail safe. A 1070 being a point of use, has a lower Cv, so not ideal for a whole house application.

    Basically this means if the cold supply to the valve fails, the valve closes off. As it can no longer mix when the cold is gone.

    The Caleffi 520 Angle Mix will fail safe. We are in the process of getting a dual listing on it 1017 and 1070. A 1070 valve cannot be set higher than 120, so a limiter on the cap is required to get the dual listing.

    The easy way to remember the differences 1070 is a higher number than 1017, so the protection level is higher.

    You will need a fairly oversized array size to tank capacity to reach 180 with flat panels, but it is possible.

    I ran Pex right to a collector once. The 1” Pex expanded to about 3” diameter before bursting. It sounded like a shot gun going off on the roof😳

    There is a video somewhere around of me demonstrating mix valve function in my shop, showing the close off function.

    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Caleffi-520516A-Submittal.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    desert_sasquatchMad Dog_2
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    Thanks @EdTheHeaterMan.

    Hmm, ok. I found one example of a 1070 and it does say that it is "designed to shut down water flow above 120°F".

    I'm still not 100% clear whether it does that in all conditions though or whether that's just what it does when it's operating as intended. Because obviously any thermostatic mixing valves designed for the proper temperature range will do that when operating as intended. Perhaps I'll have to call the company.

    If the 1070's do fail closed 100% of the time then my second question would be whether there's a reason I can't use them as a point-of-distribution thermostatic mixing valve. They seem to be intended as point-of-use valves (at least that's how the valve I linked to above is described) and so I wonder if there's a larger reason for that. Perhaps they're just all sized for a single faucet or shower and there aren't any large enough for the hot water supply for an entire home?

    If I sound paranoid, I blame the Idronics I read (I don't recall which, I read half of about three of them looking for info) which said that the point of distribution mixing valves say that they're meant to be used with point of use mixing valves, I believe precisely because of the concern about scalding. In other words, it sounded to me like they explicitly were not willing to claim that their valves would prevent scalding when they failed (though obviously if the valve was working properly it would do so). But maybe that's changed? Or it's also possible I misread or misunderstood something.

  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    @hot_rod Hah right on target. The 520 Angle Mix is exactly what I was looking for, thank you. Do you expect it'll be approved a year from now?

  • hot_rod
    hot_rod Member Posts: 22,251
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    @hot_rod Hah right on target. The 520 Angle Mix is exactly what I was looking for, thank you. Do you expect it'll be approved a year from now?

    It’s available now, works as a fail safe, just doesn’t have the second listing yet. Time and boatloads of money to ASSE is what it takes🤔
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    desert_sasquatch
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    Regarding the 180 F water thing I brought up: That's probably a bit of a tangent but most recently I'm back to thinking I'd use PV direct to heat water--in other words PV panels wired directly to resistance elements. I know I mentioned it before but I found a chart that suggests a much higher output than I was thinking would be possible for them, making them--I think--the most economical option.

    They avoid the issues with overheating (not that those issues are insurmountable) and avoid the big losses in efficiency when it's cold (though obviously any solar panel will do less well when there's less sunlight generally). Because of the latter, my thinking was that it's now practical to heat water up to 140-180 before I pump it into the heat bank. Since I'm still thinking I'd do a gravel pit heat bank under my slab (with insulation between the slab and the gravel) the higher temperature means I can store more heat using less PEX.

    Though all that said, if folks tell me that the higher temperature will degrade the PEX a lot more quickly then I might pay for more PEX and drop the top temperature down to...160? 150?

    Anyhow, that's the short version of why I'm talking about 180 degree water.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,050
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    https://www.supplyhouse.com/Resideo-Braukmann-AM101-US-1LF-3-4-Sweat-Union-Mixing-Valve-LF?_br_psugg_q=am101 Is one of many Resido (Formerly Honeywell) valves that I used for point of source. Califfe and Watts also offer them. There are so many to choose from, Sweat, FPT, Press, and push on connections. You can ask the Supply house you purchase your material from what brand they stock.

    If you like your Tap Water hotter than 120°F, you may select one with a high temperature setting of 145° and set it for 125° or 130° and lock the setting with a Hex screw on the cap.

    And they are designed to Fail Safe. I just never put it in writing on a contract. Only the "ASSE 1017" You never know what a homeowner with power tools might do after you leave the job. The normal failure of a thermostatic spring inside the valve will cause the hot side to not open at all. only the cold side, or no hot or cold from either inlet port. The ASSE standard requires that be part of the design.

    I am no lawyer and I am not an engineer. I just follow their guidance in my dealing with the public. That is why I pay them. Tell me how to stay out of trouble.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    desert_sasquatchMad Dog_2
  • Intplm.
    Intplm. Member Posts: 1,996
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    Fail-safe? Never saw any language that says fail-safe. Tempering valves I would stay away from. You probably would want something called a thermostatic mixing valve and not a tempering valve.
    Also. New shower valves have similar technology that thermostatic mixing valves have. The safety is built in.
    Mad Dog_2
  • hot_rod
    hot_rod Member Posts: 22,251
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    Regarding the 180 F water thing I brought up: That's probably a bit of a tangent but most recently I'm back to thinking I'd use PV direct to heat water--in other words PV panels wired directly to resistance elements. I know I mentioned it before but I found a chart that suggests a much higher output than I was thinking would be possible for them, making them--I think--the most economical option. They avoid the issues with overheating (not that those issues are insurmountable) and avoid the big losses in efficiency when it's cold (though obviously any solar panel will do less well when there's less sunlight generally). Because of the latter, my thinking was that it's now practical to heat water up to 140-180 before I pump it into the heat bank. Since I'm still thinking I'd do a gravel pit heat bank under my slab (with insulation between the slab and the gravel) the higher temperature means I can store more heat using less PEX. Though all that said, if folks tell me that the higher temperature will degrade the PEX a lot more quickly then I might pay for more PEX and drop the top temperature down to...160? 150? Anyhow, that's the short version of why I'm talking about 180 degree water.
    Keeping  in mind the best pv has around a 20% conversion when new. Solar thermal should run into the 50% Properly applied😉
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    desert_sasquatchMad Dog_2
  • hot_rod
    hot_rod Member Posts: 22,251
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    The nicest shower valves have both pressure and temperature balance function. Grohe for example
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Intplm.desert_sasquatchMad Dog_2
  • Kickstand55
    Kickstand55 Member Posts: 110
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    The best way to reduce the possibility, not ensure the possibility, of mixing valve failure, is to have your water tested and if needed, install suggested water conditioning equipment.
    Acidic water and minerals such as calcium, iron and manganese will foul the element, often times sending hotter water to the fixtures.
    My experience is most people do not want to go there. Too complicated, maintenance required etc. They suffer the consequences of failed devices, leaks and repair bills.
    Mad Dog_2
  • desert_sasquatch
    desert_sasquatch Member Posts: 118
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    @Kickstand55 I'll be getting what I believe is pretty good quality municipal water. But that said, I'm certainly open to conditioning the water to increase the life of components if it saves money. Who would you recommend I send a water sample to?
  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,099
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    Sqautch...there are plenty of testing labs online and probably a few you could drive to.  Thats a very good idea.  Holby (Old school mechanical valve) valves are used Extensively in NYC for close to 80-100 yrs...I believe, with an excellent track record as far as I have seen.  That would mix down anything downstream of it. Heat Timer also makes a very good electronic/digital version.  We never want scalding water at ANY fixture, but the biggest concern is in the showers/bath, where your body is immersed and you have no time to react and pull away.  Lavatories & Kitchen sinks are - if there is a concern or ADA Handicapped ♿  - safely mixed down with a very simple Tee-type mechanical mixing valve.  I understand your concern, a Solar Thermal can get terribly hot 🔥.  I've not liked how PEX got rubbery & soft the PEX got off a  Hydronic tankless coil in A large residential Peerless Gas Steamer we  Did...and that was 180-190.  Hot Rod...thats cool..I always wanted to push it to its limit and see what it would do!  I don't even Trust 95/5 solder on the joints of solar thermal piping. The melting temperature 🌡 is too close for comfort for me. I use only silver brazing rod. All of the major shower body manufacturers have addressed this problem.  If you really want to play it safe in the showers 🚿 set a mixing valve to "tempered" range  85-110.  This is a Hospital 🏥 standard.  Mad Dog 🐕 
    desert_sasquatchIntplm.
  • HVACNUT
    HVACNUT Member Posts: 5,861
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    I've worked in a few places with special needs people and there was a temperature controlled solenoid valve that was powered (or not powered) through an aquastat on the domestic hot after the mixing valve. If the aquastat sensed water too hot,  the solenoid would close and stop the hot water flow completely. 
    desert_sasquatch
  • dullknife1
    dullknife1 Member Posts: 58
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    Don’t forget to pipe that pressure/temperature relief valve in a very safe direction!!